Subtracting Fractions with Like Denominators - Khan Academy Style - 4th Grade Mathematics

Khan Academy-inspired educational content with detailed explanations for subtracting fractions with like denominators, step-by-step methods, and practice problems for 4th grade students.

Khan Academy Problems: 1 to 3
1 Basic subtraction with like denominators
Khan Problem 1: Basic Subtraction
Calculate:
\(\frac{7}{8} - \frac{3}{8}\)
Definition:

Fraction: A number representing a part of a whole, written as \(\frac{numerator}{denominator}\)

Like fractions: Fractions with the same denominator

Numerator: The top number in a fraction (parts taken)

Denominator: The bottom number in a fraction (total equal parts)

Subtraction method for like fractions:
  1. Keep the same denominator
  2. Subtract the numerators
  3. Simplify if possible
Expression
\(\frac{7}{8} - \frac{3}{8}\)
Subtraction
\(\frac{4}{8}\)
Simplified
\(\frac{1}{2}\)
Step 1: Check denominators

Both fractions have denominator 8, so they are like fractions

Step 2: Subtract numerators

\(7 - 3 = 4\)

Step 3: Keep the denominator

\(\frac{7}{8} - \frac{3}{8} = \frac{4}{8}\)

Step 4: Simplify the fraction

\(\frac{4}{8} = \frac{4 ÷ 4}{8 ÷ 4} = \frac{1}{2}\)

\(\frac{7}{8} - \frac{3}{8} = \frac{1}{2}\)
Final answer:

\(\frac{7}{8} - \frac{3}{8} = \frac{1}{2}\)

Applied rules:

• Like fractions rule: \(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)

• Simplification: Divide numerator and denominator by their GCD

• Denominator stays same: Only numerators change in like fractions

2 Subtraction with visual representation
Khan Problem 2: Visual Subtraction
Calculate:
\(\frac{5}{6} - \frac{2}{6}\)
Definition:

Visual representation: Using models or diagrams to understand fraction operations

Equivalent fractions: Fractions that represent the same value with different numerators and denominators

Expression
\(\frac{5}{6} - \frac{2}{6}\)
Subtraction
\(\frac{3}{6}\)
Simplified
\(\frac{1}{2}\)
Step 1: Verify like denominators

Both fractions have denominator 6, so they are like fractions

Step 2: Subtract numerators

\(5 - 2 = 3\)

Step 3: Maintain denominator

\(\frac{5}{6} - \frac{2}{6} = \frac{3}{6}\)

Step 4: Simplify to lowest terms

GCD(3,6) = 3
\(\frac{3}{6} = \frac{3 ÷ 3}{6 ÷ 3} = \frac{1}{2}\)

\(\frac{5}{6} - \frac{2}{6} = \frac{1}{2}\)
Final answer:

\(\frac{5}{6} - \frac{2}{6} = \frac{1}{2}\)

Applied rules:

• Like fractions rule: \(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)

• Subtraction principle: Only numerators change in like fractions

• Simplification: Always reduce to lowest terms

3 Subtraction with simplification
Khan Problem 3: Simplification Challenge
Calculate:
\(\frac{9}{12} - \frac{5}{12}\)
Definition:

Simplest form: A fraction where the numerator and denominator have no common factors other than 1

GCD: Greatest Common Divisor - largest number that divides both numerator and denominator

Expression
\(\frac{9}{12} - \frac{5}{12}\)
Subtraction
\(\frac{4}{12}\)
Simplified
\(\frac{1}{3}\)
Step 1: Confirm like denominators

Both fractions have denominator 12, so they are like fractions

Step 2: Perform numerator subtraction

\(9 - 5 = 4\)

Step 3: Keep the denominator

\(\frac{9}{12} - \frac{5}{12} = \frac{4}{12}\)

Step 4: Simplify using GCD

GCD(4,12) = 4
\(\frac{4}{12} = \frac{4 ÷ 4}{12 ÷ 4} = \frac{1}{3}\)

\(\frac{9}{12} - \frac{5}{12} = \frac{1}{3}\)
Final answer:

\(\frac{9}{12} - \frac{5}{12} = \frac{1}{3}\)

Applied rules:

• Like fractions rule: \(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)

• Simplification: Divide by GCD to get simplest form

• Final form: Always express in lowest terms

Subtracting Like Fractions: Rules and Methods
\(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)
Like Fractions Subtraction Rule
Rule 1
\(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)
Subtracting like fractions
Rule 2
\(\frac{a}{c} - \frac{a}{c} = 0\)
Subtracting identical fractions
Rule 3
\(\frac{a}{c} - \frac{0}{c} = \frac{a}{c}\)
Subtracting zero fraction
Key Definitions:

Fraction: A number representing a part of a whole, written as \(\frac{numerator}{denominator}\)

Numerator: The top number in a fraction (parts taken)

Denominator: The bottom number in a fraction (total equal parts)

Like fractions: Fractions with identical denominators

Unlike fractions: Fractions with different denominators

Equivalent fractions: Fractions representing the same value with different numerators and denominators

Simplest form: A fraction where the numerator and denominator have no common factors other than 1

Complete Methodology:
  1. Identify like denominators: Check that both fractions have the same denominator
  2. Subtract numerators: Subtract the numerator of the second fraction from the numerator of the first fraction
  3. Keep denominator: The denominator remains unchanged
  4. Simplify: Reduce to lowest terms by dividing by GCD
  5. Verify: Check that the answer makes sense in context
Tip 1: Always check if denominators are the same before subtracting.
Tip 2: Remember: only numerators change in like fractions subtraction.
Tip 3: Always simplify your final answer by dividing by the GCD.
Tip 4: If the numerator becomes 0, the result is 0.
Tip 5: Practice with visual models to understand the concept better.
Common errors: Subtracting denominators, forgetting to simplify, subtracting in wrong order.
Key notes: Denominator stays the same, only subtract numerators, always simplify.
Formulas to know by heart:

• Like fractions subtraction: \(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)

• Identity property: \(\frac{a}{c} - \frac{a}{c} = 0\)

• Zero property: \(\frac{a}{c} - \frac{0}{c} = \frac{a}{c}\)

• Simplification: Divide numerator and denominator by their GCD

Khan Academy Problems: 4 to 5
4 Subtraction with multiple steps
Khan Problem 4: Multi-Step Challenge
Calculate:
\(\frac{11}{15} - \frac{7}{15} + \frac{2}{15}\)
Definition:

Mixed operations: Performing addition and subtraction with like fractions in sequence

Expression
\(\frac{11}{15} - \frac{7}{15} + \frac{2}{15}\)
Subtract first
\(\frac{4}{15} + \frac{2}{15}\)
Add
\(\frac{6}{15}\)
Simplified
\(\frac{2}{5}\)
Step 1: Confirm all fractions have like denominators

All fractions have denominator 15, so they are like fractions

Step 2: Perform operations from left to right

\(\frac{11}{15} - \frac{7}{15} = \frac{11-7}{15} = \frac{4}{15}\)

Step 3: Continue with the next operation

\(\frac{4}{15} + \frac{2}{15} = \frac{4+2}{15} = \frac{6}{15}\)

Step 4: Simplify the final result

GCD(6,15) = 3
\(\frac{6}{15} = \frac{6 ÷ 3}{15 ÷ 3} = \frac{2}{5}\)

\(\frac{11}{15} - \frac{7}{15} + \frac{2}{15} = \frac{2}{5}\)
Final answer:

\(\frac{11}{15} - \frac{7}{15} + \frac{2}{15} = \frac{2}{5}\)

Applied rules:

• Order of operations: Work from left to right

• Like fractions rule: Only numerators change

• Simplification: Always reduce to lowest terms

5 Word problem with like fractions
Khan Problem 5: Word Problem Challenge
Sarah had \(\frac{7}{8}\) of a pizza. She ate \(\frac{3}{8}\) of the pizza. How much pizza does she have left?
Definition:

Word problems: Real-world scenarios requiring mathematical operations to solve

Initial amount
\(\frac{7}{8}\)
Amount eaten
\(\frac{3}{8}\)
Subtract
\(\frac{4}{8}\)
Simplified
\(\frac{1}{2}\)
Step 1: Identify what needs to be found

We need to find remaining pizza: Initial amount - Amount eaten

Step 2: Set up the subtraction

\(\frac{7}{8} - \frac{3}{8}\)

Step 3: Subtract the numerators

\(7 - 3 = 4\)

Step 4: Keep the denominator

\(\frac{7}{8} - \frac{3}{8} = \frac{4}{8}\)

Step 5: Simplify the result

GCD(4,8) = 4
\(\frac{4}{8} = \frac{4 ÷ 4}{8 ÷ 4} = \frac{1}{2}\)

Sarah has \(\frac{1}{2}\) of the pizza left
Final answer:

Sarah has \(\frac{1}{2}\) of the pizza left

Applied rules:

• Problem solving: Identify operation needed from context

• Like fractions rule: Only subtract numerators

• Real-world connection: Interpret mathematical result in context

Detailed Summary: Subtracting Fractions with Like Denominators
\(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)
Like Fractions Subtraction Formula
Key definitions:

Fraction: A number representing a part of a whole, written as \(\frac{numerator}{denominator}\)

Numerator: The top number in a fraction (parts taken)

Denominator: The bottom number in a fraction (total equal parts)

Like fractions: Fractions that have the same denominator

Unlike fractions: Fractions that have different denominators

Equivalent fractions: Fractions that represent the same value but have different numerators and denominators

Simplest form: A fraction where the numerator and denominator have no common factors other than 1

GCD: Greatest Common Divisor - the largest number that divides both numerator and denominator

Complete methodology:
  1. Identify like denominators: Confirm that both fractions have identical denominators
  2. Subtract numerators: Subtract the numerator of the second fraction from the numerator of the first fraction
  3. Keep denominator: The denominator remains unchanged in the result
  4. Simplify result: Reduce to lowest terms by dividing numerator and denominator by their GCD
  5. Check answer: Verify that the result makes sense in the context of the problem
Tip 1: Always check if denominators are the same before subtracting.
Tip 2: Remember: only numerators change in like fractions subtraction.
Tip 3: Always simplify your final answer by finding the GCD.
Tip 4: Practice with visual models like fraction bars to understand the concept.
Tip 5: If the result has a numerator of 0, the answer is 0.
Tip 6: Use the phrase "same bottom, subtract tops" to remember the rule.
Common errors: Subtracting denominators, forgetting to simplify, subtracting in wrong order.
Exam preparation: Practice quick identification of like fractions, memorize the subtraction rule, work on simplification.
Memory aids: "Same bottom, subtract tops" for like fractions subtraction.
Formulas to know by heart:

• Like fractions subtraction: \(\frac{a}{c} - \frac{b}{c} = \frac{a-b}{c}\)

• Identity property: \(\frac{a}{c} - \frac{a}{c} = 0\)

• Zero property: \(\frac{a}{c} - \frac{0}{c} = \frac{a}{c}\)

• Simplification: Divide numerator and denominator by their GCD

• Equivalent fractions: \(\frac{a}{b} = \frac{ac}{bc}\)

Like Fractions Subtraction Visualization: Understanding Concepts
Khan Level 6: Visualizing Like Fractions Subtraction
Compare the following like fractions subtraction operations:
\(A: \frac{5}{6} - \frac{2}{6}\)
\(B: \frac{7}{8} - \frac{3}{8}\)
\(C: \frac{9}{10} - \frac{4}{10}\)

Analysis: The chart shows the results of different like fractions subtraction operations.

  • \(A: \frac{5}{6} - \frac{2}{6} = \frac{3}{6} = \frac{1}{2}\) (Simple subtraction)
  • \(B: \frac{7}{8} - \frac{3}{8} = \frac{4}{8} = \frac{1}{2}\) (Simplifies to half)
  • \(C: \frac{9}{10} - \frac{4}{10} = \frac{5}{10} = \frac{1}{2}\) (Also simplifies to half)

Questions & Answers

Question: Why can't I subtract the denominators when subtracting like fractions? Why do I only subtract the numerators?

Answer: The denominator tells us how many equal parts the whole is divided into. Think of it like this:

  • If you have 7 apples and give away 3 apples, you have 4 apples left
  • The "type" of item (apples) doesn't change, only the quantity changes
  • Similarly, in \(\frac{7}{8} - \frac{3}{8}\), both fractions are "eighths"
  • You're subtracting 3 eighths from 7 eighths, leaving 4 eighths

Example: \(\frac{7}{8} - \frac{3}{8}\)
We have 7 parts out of 8 equal parts, and we take away 3 parts out of 8 equal parts
Result: 4 parts out of 8 equal parts = \(\frac{4}{8}\)

If we subtracted denominators: \(\frac{7-3}{8-8} = \frac{4}{0}\), which is undefined!

Question: When do I need to simplify my answer after subtracting like fractions?

Answer: You should always simplify your answer to its simplest form:

  • Find the Greatest Common Divisor (GCD) of the numerator and denominator
  • Divide both numerator and denominator by the GCD
  • The result is the fraction in its simplest form

Example 1: \(\frac{6}{8} - \frac{2}{8} = \frac{4}{8}\)
GCD(4,8) = 4
\(\frac{4}{8} = \frac{4÷4}{8÷4} = \frac{1}{2}\)

Example 2: \(\frac{9}{12} - \frac{3}{12} = \frac{6}{12}\)
GCD(6,12) = 6
\(\frac{6}{12} = \frac{6÷6}{12÷6} = \frac{1}{2}\)

Simplifying makes the fraction easier to work with and understand, and it's considered the standard way to express fractions.

Question: How do I know if a word problem requires subtracting like fractions?

Answer: Look for these key phrases and situations in word problems:

  • "How much is left?" or "What remains?"
  • "Take away" or "give away"
  • "More than" or "less than" when comparing quantities
  • Problems involving the same unit of measurement

Example: "Sarah ate \(\frac{3}{5}\) of a pizza and her brother ate \(\frac{1}{5}\). How much more did Sarah eat?"
This is a subtraction problem: \(\frac{3}{5} - \frac{1}{5} = \frac{2}{5}\)

Example: "Tom has \(\frac{7}{8}\) of a tank of gas. He uses \(\frac{3}{8}\) of the tank. How much gas is left?"
This is also subtraction: \(\frac{7}{8} - \frac{3}{8} = \frac{4}{8} = \frac{1}{2}\)

Look for the action of taking away or comparing amounts that have the same whole (same denominator).